New Scientist magazine has a feature called "The last word", in which readers' questions are answered by other readers.

Recently one reader's question asked what point on earth was farthest from the nearest sea, meaning farthest from any inlet of the world ocean. Rivers don't count, just seas, connected to the world ocean and subject to tides of the whole sea, not just seiches.

A reply was published, calling this point the "continental pole of inaccessibility (CPI)", that said the question was raised in the 1960's when builders of submarine-mounted nuclear missiles wanted to tout an ability to target any point on land from a body of water accessible to the sub fleet.

The reply stated that the point is at 46°17'N, 86°40'E, in northwest China near the Russian border, and went on to say that the nearest arm of the sea was 2848 km away at Tianjin on the Yellow Sea. It stated further, that the sub proponents neglected to mention in promotional literature that as the rockets had barely enough range to cover this distance, that "to strike the pole [of inaccessibility] a large nuclear-powered submarine would practically have to visit Tianjin docks."

If there were only one point on the edge of the sea that was that close, a farther point could be achieved by moving the "pole of inaccessibility" farther away from that point. To be limited to the 2848 km distance, there must be other shores at the same distance, in a direction at least 90° away from that of Tianjin on the Yellow Sea. In fact, if there were only one such other point besides Tianjin, then there'd still be room to go farther, along a line perpendicular to the two nearest points. So there must be a third point also constraining the CPI from being removed from the stated point.

A check with an Atlas and some distance-calculating software shows that the other two points are along the Kara Sea in Siberia, and near Calcutta on the Bay of Bengal. They are equally as close to the stated location as the port of Tianjin.

The location on Earth furthest from the ocean is located in the north-western Chinese province of Xinjiang. Within the uncertainty inherent to the definition of the coastline, two locations are proposed as Pole of Inaccessibility: EPIA1 (44°18'1"N; 81°51'31"E) and EPIA2 (45° 17' 60"N; 88° 8' 24"E). EPIA1 is equidistant 2510+10 km from Gulf of Ob, Gulf of Bengal and the Arabian Sea, and EPIA2 is equidistant 2514+7 km from Gulf of Ob, Gulf of Bengal and Gulf of Bohai (China). EPIA1 and EPIA2 are 435 and 156 km far respectively from the location popularly accepted as the EPIA (Figure 4).

The significance for the puzzle, though, is that it is necessary to be equidistant from three points on the coastline--not just one. And those three points must lie on a circle with the radius as the maximal distance to the sea, and they can't be concentrated on just one semicircle, otherwise there'd be room to go in the opposite direction.